1. Field of the Invention
This invention relates to improved catalysts for the dehydrogenation of hydrocarbons to corresponding more-unsaturated hydrocarbons, more particularly, to the production of vinyl aromatic hydrocarbons from alkyl aromatic hydrocarbons and to the production of olefins from the corresponding more-saturated aliphatic hydrocarbons.
2. The Prior Art
The vinyl benzenes and butadienes play a particularly important role in the preparation of synthetic rubbers, plastics and resins. The polymerization of styrene for example with various comonomers such as butadiene to produce synthetic rubbers is well known as is the polymerization of styrene to produce polystyrene resins.
Styrene and butadiene are typically produced from ethyl benzene and butylene, respectively, by dehydrogenation over solid catalysts in the presence of steam, and at temperatures ranging from 500.degree. to 700.degree. C. The class of catalysts found to be the most effective for this process is a potassium oxide (carbonate) promoted, chromium oxide stabilized, iron oxide material. Considerable research has gone into attempting to improve the activity and selectivity of this class of catalysts. Any improvement which results in either increasing the selectivity (moles of desired product produced per mole of reactant reacted) or the conversion (moles of reactant reacted per mole of starting material) without lowering the other is economically attractive since the result is that the yield (moles of desired product produced per mole of reactant) of the product has been increased. Any increase in the numerical value of the yield results in a more efficient operation with more reactant being converted into the desired product. In commerical operations many of which produce millions of pounds of product per year, an increase of only 1 or 2 percentage points in the selectivity or yield can result in a substantial net increase in the plant production or a substantial savings of starting materials.
The addition of vanadium pentoxide is known to improve the selectivity of the above described iron-chromium-potassium oxide catalysts. Such catalysts containing vanadium pentoxide were desclosed in U.S. Pat. No. 3,361,683 to W.R. Gutmann, issued Jan. 2, 1968 or U.S. Pat. No. 3,084,125 to F.J. Soderquist, issued Apr. 2, 1963.
Addition of cobalt to a typical iron-chromium-potassium oxide catalyst has been disclosed in U.S. Pat. No. 3,291,756 to R.S. Bowman, issued Dec. 13, 1966. In copending applications Ser. Nos. 740,262 now U.S. Pat. No. 4,052,338, and 740,264 filed Nov. 8, 1976, the addition of cobalt and vanadium to iron-chromium potassium oxide catalysts is taught.
Commercial iron-oxide based potassium catalysts typically utilize chromium oxide as structural stabilizers. P. H. Lee in CATALYSIS REVIEWS 8 (2) at page 295 (1973) notes that removing chromium oxide from alkali-promoted iron oxide catalysts results in a very rapid loss in activity in only a few hours. It has been found that the addition of cobalt and vanadium compounds to potassium promoted iron oxide based catalysts results in catalysts which do not exhibit such rapid activity loss.